USING MICROSCOPES TO MEASURE FRICTION

Researchers are using Atomic Force Microscopes to look into the
atomic origin of frictional forces

1993 LBL Highlights

Topographic map of a gold surface at one-angstrom
resolution showing atoms as small protrusions.(Top picture)
Map of the same surface showing the drag force of friction as
the bidirectional AFM tip rocks over the gold atoms.
Lightened areas reveal high friction; darker areas show low
frictional drag.(Bottom picture)

Exploring the mystery of friction, LBL scientists have devised an
instrument that records frictional forces while mapping a surface at an
atomic scale.
A scientific team led by Miquel Salmeron of LBL's Center for Advanced
Materials has developed a unique bidirectional Atomic Force Microscope
(AFM) that can simultaneously map surface topography and measure the
"dragging" or frictional force of the tip as it scans across a
surface.

The instrument opens a window that will allow researchers to investigate
friction, adhesion, wear, and the effect of different lubricants.

When two surfaces come in contact with each other, the atoms do not roll
across one another like balls. Instead, they have chemical bonds and
clouds of electrons (atomic orbitals) which determine the maximum drag.
The properties of these orbitals and their relative orientations
determine the strength of the frictional forces.

What factors increase or decrease these frictional forces? If we put oil
or water molecules on the surface, how are the frictional forces altered?
The bidirectional AFM will help us begin to understand such basic
questions.

Much like a phonograph player, the bidirectional AFM maps a surface by
scanning a very sharp tip across a sample, but with a force so slight
that single atomic bonds are not broken during the scan. A conventional
AFM measures surface topography as the tip moves up and down over
individual atoms. To measure drag, the bidirectional AFM incorporates
features which also allow a computer to record the sideways or rocking
motion of the flexible, cantilevered tip.

The instrument allows scientists to study wear at an atomic scale,
increasing the compressive force on the tip and measuring how much force
is required to dislodge single atoms. Researchers also can measure
whether frictional forces are uniform or if they vary based upon a
pattern that is correlated to the atomic topography of the surface.

In biology, chemistry, and physics, practical questions are addressed and
resolved based on research at the atomic scale. But in the study of the
mechanical properties of surfaces, researchers have not yet made that
leap. This instrument gives scientists the capability to explore the
mechanisms underlying lubrication, adhesion, and the interaction between
surfaces. Ultimately, what we learn should lead to better lubricants
which extend the lives of moving parts and improve the energy
efficiencies of mechanical devices.